Silicon nitride based sintered bodies (silicon nitride sintered bodies) which have excellent heat resistance, wear resistance, etc. have been used as materials for various types of cutting tools.
However, silicon nitride is difficult to sinter, so usually a sintering aid is used for sintering. Since use of a large amount of the sintering aid deteriorates the performance of a sintered body, the amount of the sintering aid is preferably small within a range of allowing sintering.
Accordingly, various proposals have been presented as in following Patent Documents 1-5 concerning, for example, types of aids and reduction of the use amount of aids, in order to achieve an improved performance.
Patent Document 1 discloses a silicon nitride sintered body for cutting tool which is obtained by using extremely reduced amounts of aids, such as oxides of Mg, Zr and Ce, and thereby has an especially excellent wear resistance.
Patent Document 2 discloses a technique of volatilizing an aid in an outermost surface of a sintered body to reduce an aid amount to less than an aid amount inside to thereby achieve an improved wear resistance.
Patent Document 3 discloses a technique of forming α-sialon in a surface of a silicon nitride sintered body by heat treatment to thereby achieve an increased surface hardness and an improved wear resistance.
Patent Document 4 discloses a technique of sintering in a SiO gas atmosphere to thereby reduce roughness of a sintered surface of a sintered body and form a hard phase having a Vickers hardness of 16 GPa or more from an outermost surface to a depth of 10 μm of the sintered body.
Patent Document 5 discloses a technique of defining rare earths, Mg, Al as grain boundary forming components and a total amount, and thereby improving thermal shock resistance.
Patent Document 1: Japanese Patent No. 3550420
Patent Document 2: Japanese Unexamined Patent Publication No. 2002-12474
Patent Document 3: Japanese Unexamined Patent Publication No. 9-183667
Patent Document 4: Japanese Unexamined Patent Publication No. 8-323509
Patent Document 5: Japanese Unexamined Patent Publication No. 11-268957